1. Field of the Invention
The present invention relates to a reference electrode which will give a reference electric potential for a potentiometric system for detecting ionic matters in a solution. Specifically, the said reference electrode has a nano-porous junction material and a liquid electrolyte containing a water soluble organic compound with the molecular size and boiling point both greater than water.
2. Description of the Related Art
The ion-sensitive field effect transistor (ISFET) has many advantages, such as the miniaturization and is widely used to potentiometrically measure the concentration of specific ions in liqueous samples in household health care or environmental monitoring. To obtain the relationship of the electrical potential and the concentration of the detected matters correctly, a reference electrode offering a stable reference electric potential is needed in the ISFET potentiometric system. However, compared with the size of the ISFET, the size of a traditional Ag/AgCl glass reference electrode is bulky and while it is not being used, it must be stored in saturated KCl solution to prevent the evaporation of the aqueous electrolyte in the reference electrode. Moreover, the traditional Ag/AgCl glass reference electrode is easily polluted by the contaminants in the tested solution through the junction material if the tested solution is not properly addressed prior to the measurement, which results in the shift of the reference electric potential and causes deadly deviations. Therefore, the functions and features of the traditional Ag/AgCl reference electrode are not suitable for an ISFET potentiometric system.
Although much effort has been dedicated to solve some problems of the traditional Ag/AgCl glass reference electrode mentioned above, the achievements are limited. For example, in order to reduce the dry of the traditional reference electrode in the low humidity, Okada et al. suggested adding NH4NO3, LiCl, agar into the saturated solution of KCl to prepare a gelatinous electrolyte of the Ag/AgCl reference electrode (U.S. Pat. No. 5,334,305). Although NH4NO3 and LiCl both are hydrophilic, the achievement of improving water loss in the reference electrode is limited.
Moreover, to miniaturize the size of the Ag/AgCl reference electrode, a strategy of silicon-based multilayer technology is widely adopted. In this case, a metal layer such as silver is coated on a ceramic or silicon substrate, and then the surface of the silver layer is oxidized to form an Ag/AgCl complex layer by electrochemical approach or a haloid oxidizer such as ferric chloride. Furthermore, a gelatinous electrolyte and a protective polymeric membrane are coated on the substrate in sequence. Finally, the device is sealed except the polymeric membrane. The performance of the protective polymeric membrane is to reduce the fast leaching out of the electrolytic salts in the electrolyte and prolong the life span of the reference electrode. The higher the leaching rate of the salts in the electrolyte, the shorter the lifespan of the described reference electrode. Reversely, if the leaching of the salts in the electrolyte is too slow, the response of the reference electrolyte will spend a lot of time. In practical, this kind of reference electrode usually needs a preconditioning time to get a stable electric potential by dipping the tested solution before measurement. For example, H. J. Lee et al. published a PU membrane which has been added cation and anion salts as a protective membrane (Anal. Chem., 70 (1998) 3377; Sens. Actuators B, 34(2000) 8; Proc. IEEE, 91 (2003) 870). However, this kind of reference electrode needs one hour of preconditioning time and was impractical. Although Ha et al. further modified the PU membrane with hydrophilic polymer and cellulose acetate to increase the permeability between water and the electrolyte, the reference electrode still needs 1.5-3.5 minutes of preconditioning time and the lifespan of the reference electrode is only 20-100 minutes (Analytica Chimica Acta, 549 (2005) 59). Therefore, according to the present technology, a reference electrode with a small size, a long lifespan, stable electric potential, and easy storage, has yet to be obtained.